Thanks for the link. I missed that one.
I'll be in Playa del Carmen and using recreational limits. I believe most dives bottom out at 90 feet. I'll also be on Nitrox 32 or 36 as sites allow.
Bert,
Whether you are using Nitrox 32, Nitrox 36 or Air makes no difference when you are exposed to CO at pressure.
Subcooled stated:
- The pressure (of breathing gas and CO gas) goes up with depth.
This is only partially correct. It is the gas partial pressures that matter here, not the actual pressure of the surrounding water.
This shows the March 1970 US Navy Diving Manual's diagram to explain partial pressures. Note that it is directly related to the absolute pressure, starting with one atmosphere (14.7 psi) at the surface. You are talking of diving to four atmospheres absolute pressure, which is 4 x 14.7 psi or 58.8 psi. 3 ppm, which would be indicated on the coot as a warning, represents 0.000003 x 100 or 0.0003% CO in the air, at the surface (surface pressure).
3/1,000,000 = 0.000003, or 3 x 10^-6
0.000003 = X/100 = %
X = 0.000003 x 100 = 0.0003%
But at depth of 99 feet, you need to multiply that by 4, so that 0.0003% x 4 = 0.0012% CO. If for some as yet unknown reason you go to 132 feet, which is 5 atmospheres absolute pressure, you would do the same calculation and get 0.0015% in the breathing mixture.
Now, let's see what the current
U.S. Navy Diving Manual states about carbon monoxide.
2-10.8 Carbon Monoxide. Carbon monoxide (CO) is a colorless, odorless, tasteless, and poisonous gas whose presence is dif cult to detect. Carbon monoxide is formed as a product of incomplete fuel combustion, and is most commonly found in the exhaust of internal combustion engines. A diver’s air supply can be contaminated by carbon monoxide when the compressor intake is placed too close to the compressor’s engine exhaust. The exhaust gases are sucked in with the air and sent on to the diver, with potentially disastrous results. Carbon monoxide seriously interferes with the blood’s ability to carry the oxygen required for the body to function normally. The af nity of carbon monoxide for hemoglobin is approximately 210 times that of oxygen. Carbon monoxide dissociates from hemoglobin at a much slower rate than oxygen.
- 3-5 1 1 Causes of Hypoxia. The causes of hypoxia vary, but all interfere with the normal oxygen supply to the body. For divers, interference of oxygen delivery can be caused by:
- ...Carbon monoxide poisoning. Carbon monoxide interferes with the transport of oxygen by the hemoglobin in red blood cells and blocks oxygen utilization at the cellular level.
Further in the manual, we find this:
- 3-5 8 1 Causes of Carbon Monoxide Poisoning. Carbon monoxide is not found in any signi cant quantity in fresh air. Carbon monoxide poisoning is usually caused by a compressor’s intake being too close to the exhaust of an internal combustion engine or malfunction of a oil lubricated compressor. Concentrations as low as 0.002 ata (2,000 ppm, or 0.2%) can prove fatal.
- 3-5 8 2 Symptoms of Carbon Monoxide Poisoning. The symptoms of carbon monoxide poisoning are almost identical to those of hypoxia. When toxicity develops gradually the symptoms are:
- Headache
- Dizziness
- Confusion
- Nausea
- Vomiting
- Tightness across the forehead
When carbon monoxide concentrations are high enough to cause rapid onset of poisoning, the victim may not be aware of any symptoms before he becomes unconscious.
Carbon monoxide poisoning is particularly treacherous because conspicuous symptoms may be delayed until the diver begins to ascend. While at depth, the greater partial pressure of oxygen in the breathing supply forces more oxygen into solution in the blood plasma. Some of this additional oxygen reaches the cells and helps to offset the hypoxia. In addition, the increased partial pressure of oxygen forcibly displaces some carbon monoxide from the hemoglobin. During ascent, however, as the partial pressure of oxygen diminishes, the full effect of carbon monoxide poisoning is felt.
Now note that this 0.002 at a, 0.2%, is at the surface, and because of the gas laws the partial pressure changes with the depth. So what would be the potentially fatal level in breathing gas at 99 feet? Well, that can also be calculated:
0.2% at surface / 4 atmospheres absolute = 0.05% at 99 feet
Also, the more time spent breathing CO, the more red blood cells that are affected. Here's the U.S. Navy Diving Manual's standard for breathing air:
DIVER’S BREATHING GAS PURITy STANDARDS
4-3.1
Diver’s Breathing Air. Diver’s air compressed from ANU or certi ed diving system sources shall meet the U.S. Military Diver’s Breathing Air Standards contained in Table 4-1.
Table 4-1. U S Military Diver’s Compressed Air Breathing Purity Requirements for ANU Approved or Certi ed Sources.
Constituent
Speci cation
Oxygen (percent by volume)
20–22%
Carbon dioxide (by volume)
1,000 ppm (max)
Carbon monoxide (by volume)
20 ppm (max)
Total hydrocarbons (as CH4 by volume)
25 ppm (max)
Odor and taste
Not objectionable
Oil, mist, particulates
5 mg/m3 (max)
I hope this helps.
SeaRat
John C. Ratliff, CSP, CIH, MSPH
NAUI 2710 (long ago retired)
